Water-absorbent resins have been widely used in various fields, including hygienic materials such as sanitary napkins, incontinence pads and disposable diapers, horticultural materials such as water retaining materials and soil improvers, and industrial materials such as water blocking materials and dew catchers. As the water absorbent resins, for example, hydrolysates of starch-acrylonitrile graft copolymers, neutralized products of starch-acrylic acid graft copolymers, saponified products of vinyl acetate-acrylic acid ester copolymers, crosslinked products of partially neutralized polymers of acrylic acid, and the like have been known.
Usually, the water-absorbent resin usable in an absorbent material of hygienic material applications has been demanded to be excellent in various properties such as water-retention capacity (water-absorption capacity), water-absorption capacity under load, water-absorption rate, and gel strength. So far, techniques remarking, for example, high water-retention capacity, excellent water-absorption rate and the like among the above-mentioned properties have been studied.
In addition, recently, water-absorbent resins have been demanded to be excellent in properties suitable in the production of hygienic materials, for example, flowability under moisture absorption. Since the water-absorbent resins absorb moisture in the atmosphere, and are likely to cause adhesion to a metal plate or aggregation between particles, a solid adhesion is likely to be formed in a production machine of hygienic materials (for example, a drum former), so that frequent cleanings would be necessitated, thereby making it likely to lower the productivity of the hygienic materials. Also, when a water-absorbent resin in an aggregated state due to moisture absorption is used in the hygienic materials, there is a possibility of worsening the properties of the hygienic materials such as an increase in leakage or re-wet caused by gel blocking.
Conventionally, in view of the above-mentioned problems, a variety of techniques have been proposed in order to improve various properties of the water-absorbent resin. For example, a method including adding an organic carboxylic acid ester of a polyvalent alcohol to a water-absorbent resin, and heat-treating the mixture (see, Patent Publication 1), a method including adding a ketal compound, an acetal compound or an alkyl ether of a polyvalent alcohol to a water-absorbent resin, and heat-treating the mixture (see, Patent Publication 2), a method including adding a specified silicone surfactant to a water-absorbent resin, and surface-treating the water-absorbent resin (see, Patent Publication 3), a method including adding a polyvalent metal to a water-absorbent resin, and surface-treating the water-absorbent resin (see, Patent Publication 4) and the like have been reported. In addition to the above, as a method using a hydrophobic compound such as a surfactant, in which a reversed phase suspension polymerization is employed as a core reaction, a method using an oil-soluble cellulose ester or cellulose ether as a dispersant (see, Patent Publication 5), a method using a sucrose fatty acid ester (see, Patent Publication 6), and the like have been reported.